The following describes a prior art, inband, messaging system (for example, voice mail system) interface a private branch exchange ("PBX") having a DID (Direct Inward Dialing) feature, which PBX is connected to a central office ("CO") over DID lines. The normal sequence of actions for call setup in this system occurs as follows. Step 1: after an incoming seizure, the PBX receives address information from the CO. The address information takes the form of dial pulse or DTMF digits (the dial pulse or DTMF digits are transmitted from the CO to the PBX in accordance with the EIA RS-464 standard) and the address information identifies a called station, i.e., local extension at the PBX. Step 2: After the PBX receives the address information, and before the PBX sends an answer signal to the CO, the PBX returns a Call Progress tone signal to the calling party. The Call Progress tone signal depends upon the status of the called station at the PBX and typically is one of the following: Busy Tone, Reorder Tone, or Ringback Tone. Step 3: Whenever a call is answered by the called station, the PBX sends an answer signal to the CO by applying an appropriate voltage to the tip conductor of the DID circuit. At this time, a voice path is established between the calling and called parties and the CO starts billing the calling party for the call. Step 4: If the called station does not answer (this may be due a number of reasons such as, for example, immediate forwarding, busy, no answer, and so forth), the PBX, if programmed to do so, forwards the call to a messaging system such as a voice mail system ("VMS") over specific lines. Whenever the VMS answers the forwarded call, the PBX transmits the address information (for example, in the form of DTMF digits) identifying the original called station inband over one of the specific lines connecting the PBX and the VMS. At the same time, the PBX sends an answer signal to the CO by applying an appropriate voltage to the tip conductor of the DID circuit to notify the CO of the call answer. As a result, the CO considers the call to be established and the CO starts to bill the calling party.
As one can appreciate from the above, whenever a call gets forwarded to the VMS, the PBX outpulses address information (for example, DTMF digits) identifying the original called station as soon as the VMS answers the call. However, in some situations, it is desirable that the VMS not answer the call if it is intended for certain predetermined called (for example, resources are not available to service the call and in particular, a facsimile resource is not available). In such a situation, the VMS is programmed to drop the call once it recognizes that the address information refers to one of the predetermined called. A problem occurs in that the VMS needs to answer the call to collect the address information. Then, in response to that answer, the PBX notifies the CO, which notification results in billing. This billing occurs even if the VMS drops the call immediately upon recognizing that it should not have answered the call in the first place. As a consequence, a calling party will be billed for a call which he/she believes was never answered.
The solution for this problem in the prior art, is to have the VMS inform the calling party that the call cannot be handled at that time and then to have the VMS hang up. However, this still results in billing.
In light of the above, there is a need in the an for a method of interfacing a PBX to a messaging system such as a VMS wherein the PBX will send an answer signal to a CO only if the VMS will call.